Many different types of air rings have been proposed in the past, see for example U.S. Pat. No. 5,804,221 (Planeta et al.) issued Sep. 8, 1998, the contents of which are hereby incorporated herein by reference, which describes controlling air flow at different circumferential locations to adjust the gauge (thickness) of the extruded film. Another proposal is described in U.S. Pat. No. 6,739,855 (Randolph et al.) issued May 21, 2004. However, because of ever increasing need for high quality film made from different plastic materials with different physical properties, there is a requirement for improved airflow control at different circumferential locations.
It is therefore an object of the invention to provide an air ring which enables air flow at different circumferential locations to be controlled in an improved manner.